The present invention relates to field mounted industrial process control instruments and, more specifically, to a start up circuit for DC powered field mounted industrial process control instruments.
Industrial process control instruments are used in industry to monitor and control processes. A field mounted instrument, such as a transmitter, monitors a process variable such as temperature, flow rate, level or pressure. A field mounted instrument is powered by an external power supply which supplies power over a process control loop or over a separate pair of wires which are coupled to the instrument. A voltage regulator converts the instrument's input power to a regulated voltage level that can be used to operate the internal electronics of the instrument. Some field mounted instruments, such as magnetic flow meters and radar level gauges, require relatively high input power to operate, on the order of ten watts. To most efficiently use the available input power, designers of field mounted instruments are increasingly using switching regulators to regulate the voltage level.
A voltage regulator, and especially a switching voltage regulator, may require more current to begin its operation than it does to maintain its operation after the field mounted instrument is powered-up. The current that the field mounted instrument takes to begin operation, or start up, is referred to as inrush current.
Well designed external DC power supplies include current limit or overload circuits which limit the amount of power that they can put out to an instrument. The current limit or overload circuits prevent the power supply from experiencing an overload situation which may cause the power supply to have a component failure and stop working. These current limit or overload circuits may be sensitive enough to trip on high output power requirements that occur for relatively short periods of time, such as on start up of a magnetic flow meter or radar level gauge. Often, a larger power supply is needed to handle the inrush current than would otherwise be required during normal operation, which adversely increases the cost of the power supply.
Improved start up circuits are desired for field mounted instruments to allow the instruments to be used with external DC power supplies having a wide range of current limit or overload protection circuits.